Hydrogen has many industrial uses. For example, there exists a continuing need for hydrogen to treat high sulfur content fuels. In addition hydrogen is also seen as a potential replacement for fossil fuels that would otherwise be used in powering motor vehicles.
Gasification is seen as an environmental friendly process that can be used to convert carbonaceous materials, such as coal, petroleum or biomass into a synthesis gas, namely, a gas that contains hydrogen and carbon monoxide. With respect to the generation of hydrogen, the gasification of coal is extremely attractive, given recent price increases for natural gas that is used in the generation of hydrogen through steam methane reforming.
The carbonaceous material is reacted at high temperatures with oxygen addition within a gasifier to produce the synthesis gas. For example, in one type of gasifier that is used in the gasification of coal, the coal is pulverized and fed into the gasifier. The pulverized coal is heated and volatiles are released creating a char. Volatile products and some of the char is reacted with oxygen to form carbon dioxide and carbon monoxide. The char also reacts with carbon dioxide and steam to produce the carbon monoxide and hydrogen. In addition, carbon monoxide and steam also react in a known water-gas shift reaction to produce carbon dioxide and additional hydrogen.
Gasifiers are integrated with processes that generate steam to power steam turbines, utilize the synthesis gas to power gas turbines and also, to generate hydrogen. For such purposes, the synthesis gas generated by the gasifier is processed in a synthesis gas processing system in which additional hydrogen is produced in shift converters in which the synthesis gas undergoes catalyzed water-gas shift reactions. Since a water-gas shift reaction is an exothermic process, the shifted streams are cooled by heat recovery steam generators that can produce export steam to power the steam turbines. The shifted stream that results from the stages of shift conversion are then passed through an acid gas removal unit in which any sulfur species and carbon dioxide are separated from the shifted stream. Typically this is a physical absorption process that is conducted within absorption columns. The resulting purified synthesis gas is then introduced into a pressure swing adsorption unit in which the hydrogen product is separated from the purified shifted stream. The resulting tail gas can be recompressed to be further processed in pressure swing adsorption units to produce additional hydrogen.
Although for all the reasons given above, gasification and combined cycles as described above that utilize gasification are attractive processes, gasification and associated combined cycles are only beginning to be employed and have not found widespread use. The principal reason for this is that gasifiers are new and very expensive facilities that are believed to be only about 85 percent reliable with respect to the supply of hydrogen. Customer required reliability for hydrogen supply is typically above about 98 percent. In addition, gasification facilities take a long time to construct. For all of these reasons, gasification has not replaced the more traditional method of generating hydrogen, namely, steam methane reforming.
As known in the art, in steam methane reforming, natural gas and/or a refinery off-gas is introduced into a hydrotreater to hydrolyze the sulfur species to hydrogen sulfide. Hydrogen sulfide is then removed in a bed that contains zinc oxide or other material that has sulfur removal capability. Steam is added to the resultant purified natural gas and reactant mixture is introduced into reformer tubes located within a furnace as part of a steam methane reformer. The steam methane reformer is tired by burners that burn part of the natural gas and some tail gas produced by the separation of hydrogen. The combustion is supported by air. The flue gases are used in a convective section of the steam methane reformer to produce the required steam. Steam is also produced When the reformed stream leaving the reformer tubes are cooled. Excess steam is exported. The resultant reformed stream is then shifted in a shill conversion unit to produce additional hydrogen and the hydrogen product is separated from the shifted stream in a pressure swing adsorption unit.
As will be discussed, among other advantages, the present invention provides a method of producing hydrogen from a synthesis gas stream generated by a gasifier in a manner that allows for a greater reliability in the supply of hydrogen and therefore, a lower financial risk in constructing the gasification facility by integrating a steam methane reforming system into the gasification facility.